In many radio transmission systems, for instance time-divided so-called TDMA-systems, it is important that transmitter and receiver are well synchronized. It is also highly important that the local frequency generator of the receiver is locked very accurately to the transmitter frequency, particularly in the case of coherent transmission systems. An arrangement of apparatus for such a radio-transmission system is described in the article "Radio Test Performance of a Narrowband System" by Sjernvall, Hedberg and Ekemark, published in IEEE Vehicular Tech. Tampa, Fl., U.S.A., Jun. 1987.
The frequency error in a receiver where the receiver frequency has been at least roughly set is estimated in a frequency-error calculating device, and a control processor generates a correction signal for each new transmitted signal- sequence in accordance with the size of the correction obtained during the preceding signal sequence and in accordance with the estimated frequency error. The correction signal is sent to a controllable local oscillator, according to the aforesaid article a so-called frequency synthesizer, which is constructed to generate mixing signals, the frequencies of which are corrected in dependence on the control-processor signal. This signal is digital and in practice is converted to analog form in a digital/analog converter before being sent to the local oscillator. Consequently, the number of stages in which the oscillator output frequency can be adjusted is dependent on the number of binary bits which the converter is able to convert. When a relatively small and simple converter is used, either the maximum frequency swing from the local oscillator will be small or the frequency stages will be relatively large.
The transmitted signal can be subjected to interference in systems of the aforesaid kind. This interference or disturbance may, for instance, be in the form of an added noise-signal or in the form of multiple-path propagation due to repeated reflection of the signal against buildings, mountains etc.. This is often the case in mobile radio transmissions, as is described in an article published in the Norwegian technical journal Telektronikk Nr 1, 1987, Torleiv Maseng and Odd Trandem: "Adaptive digital phase modulation". The article describes a coherent receiver which incorporates an equalizer in the form of an adaptive viterbi-analyser. The transmitted signal comprises periodically recurring signal sequences which include a synchronizing sequence and a data sequence as mentioned in the aforegoing. The signals are frequency-mixed and converted in an analog-digital converter, in the aforedescribed manner, and stored in a memory. The Viterbi-analyzer is adapted to the prevailing transmission characteristics of the channel with the aid of the synchronizing sequence, these characteristics being determined primarily by said multiple path propagation. The data sequence is analyzed in the Viterbi-analyzer for the purpose of extracting the content of the original transmitted signal.
An article in IEEE/IEICE Global Communications Conference, Tokyo, Nov. 15-18, 1987 by Franz Edbauer: "Coded 8-DPSK Modulation with Differentially Coherent Detection An Efficient Modulation Scheme for Fading Channels" describes a radio receiver equipped with a Viterbi-analyzer. This analyzer assists in controlling the frequency of a received signal. According to this article, the Viterbi-analyzer is set permanently and cannot be adapted to handle variations in channel conditions for instance.
A more detailed description of the Viterbi-algorithm applied in the Viterbi-analyzer is given in Richard E. Blahut: "Theory and practice of error control codes", chapter 12, Addison-Wesley 1983.